Combustion of coal, industrial waste, domestic waste, oil, peat, biomass etc. produces flue gas that contains dust particles. Emission of the dust particles to ambient air needs to be kept at a low level and electrostatic precipitators (herein after referred to as “ESP”) are the most widely used equipment to precipitate the dust particles suspended in the flue gas. To obtain the optimum collection efficiency of an ESP, the flue gas entering it from the inlet duct must be uniformly distributed over the ESP's entire cross section. An inlet transition nozzle is used at the entrance to reduce the gas velocity. The gas flow is then evenly distributed in the ESP by gas screens placed at the inlet. After the gas screens, the flue gas passes along the length of the ESP through passage between the electrodes, which are stacked in parallel along the width of the ESP. There are two types of electrodes namely, collecting electrodes and discharge electrodes that are placed in alternate fashion. Different sizes of collecting electrodes are used depending upon the design and the size of the ESP. The gaps between two collecting or emitting electrodes are standardized in ranges from 250 to 600 mm. The set of electrodes are grouped in so-called fields which are arrangements of bus sections perpendicular to the gas flow that are energised by one or more high voltage power supplies. The smallest portion of the ESP which can be independently energised is called a bus section. The charged dust particles between the discharge and the collecting electrodes are attracted by and collected on the collecting electrodes plates. The collecting electrode plates are occasionally rapped to make the collected dust release from the plates. Subsequently the dust falls down into the hoppers from which it is transported for further use or disposal. The dust free gas is then emitted to the ambient air via a stack.
In order to evaluate the uniformity of the distribution of the flue gas in an ESP, a ‘Gas Distribution Test’ is generally conducted inside the ESP. In such a test, the flue gas velocity is measured over the entire cross section of the ESP and then the coefficient of variant ‘CV’ is calculated from the velocity values to represent the flow variation in the ESP statically. This test is conducted offline (with air) and conventionally it is done manually by person(s) who take(s) the measurement of the air velocity over the ESP cross section. The necessary airflow for the measurement is generated in the ESP using an Induced Draft (ID) fan. The person then compiles all the data to calculate the CV. Depending on the size of ESP, this conventional way of measurement can take up to 8 hours for completing the test for two persons. This time includes the time taken for manual measurement, feeding data to the computer, compilation and reporting of the result.
Inside of an ESP, the space available for access/movement of persons is generally either between two fields or between the screen plates and the field. Access can be made either from the roof side or from the hopper side of the ESP depending upon its design. In some ESP, a horizontal ladder is installed for walking between the fields. For some other designs there are even no walkways. Most of the ESP has a manhole opening for the entry which is rather small.
While conducting the gas distribution test, all manholes are closed to avoid leakage of air from outside. The person needs to carry a light into the ESP for illumination. As gaps are very small, working conditions are very difficult for manual work. Large ESP can be up to 15 m high and for measurements the operator has to climb at such heights either through ladders in small space or use scaffoldings, which are dangerous from the safety point.
Additionally the inside of the ESP is very dusty due to dust from the flue gas which remained stuck and deposited on the various ESP components.
Process of taking reading manually is very long and monotonous. As large ESP generally have quite a high number of collecting electrodes, the total number of measurements is also a high figure.
Accuracy some times is not the best as the present method cause human fatigue. In very large ESP, to keep the efforts reasonable, measurements are done at lesser measurement points (generally by skipping alternate points). This affects the quality of results adversely. Collection of data is not accurate as manually person measuring the data from heights has to record with the help of another person standing down by hear say. This lead to sometimes either wrong recording of data or few missed points while recording. In few ESP designs, where human access is difficult due to small gaps, direct measurement of gas distribution is even not feasible.
Also the dust from the flue gas which remained stuck on the components and the walls of the ESP makes the movement much more difficult for operators. This dusty environment also poses a health risk to the operator which increase with residence time of operator in the ESP.
For the fore going reasons, there is a need for a method for carrying out measurement of gas distribution in an ESP in fully safe, fast and accurate manner and a system for successful implementation of the method.